ARGON ION SLICING (ArIS) OF MINERAL AND ROCK SAMPLES: A NOVEL TOOL TO PREPARE LARGE ELECTRON TRANSPARENT THIN FILMS FOR TEM USE

Introduction: Argon Ion milling is the basic conventional means used since decades when speaking of sample preparation for TEM analysis. Applied to earth and planetary samples (usually on hand as thin sections) the shortcomings are (a) nearly impossible site specific thinning and (b) only small electron transparent areas obtained. A new technique named FIB (Focused Ion Beam) developed over the past years overcame several of these draw backs [1]. However, FIB TEM sectioning remains time consuming and expensive. Furthermore the electron transparent area obtained is in most cases restricted to about 20μm x 5μm with a typical thickness of less than 100 nm and a major part of the area sampled is not seen in advance since it is located perpendicular to the sample surface. A novel alternative approach to site specific preparation for earth and planetary samples is presented here using an JEOL EM-09100IS Ion Slicer. Argon Ion slicing (ArIS) is based on a shadowing technique [2] and leads to high quality, homogenous, damage free, large electron transparent areas of up to 100 x 200 μm (20,000 μm) even for complex polyphase samples. Sample pre-treatment: Although any conventional thin section is suitable the use of removable glues (e.g. crystal bond or comparable) is recommended. The area of interest can be identified by light or electron microscope. The best results will be obtained using an ultrasonic drill to cut out a rectangular area (2mm x 3mm). Applying this technique the margins produced are straight and intact, which is a necessary precondition for later successful ArIS. After cutting the rectangle out of the specimen a half circular copper grid is glued onto its surface. In most cases subsequent acetone treatment is sufficient to separate the sample from the glass part of the microscope slide. Samples initially not intended for TEM analysis may require longer acetone in some cases even chloroforme treatment to remove the glue. Optimised thinning: The sample rectangle with its attached copper grid is mounted into the Ion Slicer sample holder (JEOL Ion Slicer) which had been slightly modified to avoid further use of any glue. The sharp (typically 30 to 100 μm thick) edge of the sample is partially shaded by the sharp edge of a copper mask belt (10 μm) mounted above the sample. The Ion beam lies in the same plane as the sharp edges of sample and belt do. In this respect the tilt angle determines the final size of the thinned area obtained. The beam can be tilted in various angles from zero (parallel) to 6°. The slicing process alternates between front and backside whereas side change intervals are variable as are accelerating voltage and slicing time. Streaks, artefacts produced during slicing on the sample surface are minimized by a “swing-mechanism” rocking the sample stage slightly in the plane of the ion beam. With a tilt angle of 1.5° the largest electron transparent areas were obtained. According to conventional Argon Ion milling an acceleration voltage 4.5 kV with an Argon flow rate of 7.8 was chosen to balance a fast slicing rate and minimum sample damage (amorphisation, phase changes, etc.). Applying theses conditions a large “lagoon shaped” area is obtained (Fig. 1). The thinned region in total covers an area more than 500 x 500 μm (250,000 μm) with a thickness of less than 1 μm. Like in conventionally thinned samples a small hole is generated in the thinnest region of the sample. ArIS produces very large electron transparent areas surrounding the small hole.